1. Field of the Invention
The present invention relates to a predictive coding system for coding a differential value between an input value and a prediction value.
2. Related Background Art
Hitherto, as a high efficient coding system which is used in the case of transmitting data obtained by sampling a signal having a large information amount such as an image signal or the like, for instance, a differential PCM (Pulse Code Modulation) system (hereinafter, simply referred to as a DPCM) has generally been well known. The DPCM is the system in which a value of a sample point to be at present coded is predicted from the values of the sample points which have already been coded and the difference (prediction error) between the predictive value and the inherent value is coded. For a signal such as an image signal or the like in which a correlation among the; values at the close sample points is large, highly-efficient coding can be executed by performing the non-linear digitization in consideration of a deviation of the generation distribution of the prediction error signals or the like.
FIG. 1 shows a schematic arrangement of a conventional predictive coding circuit. An explanation will now be made with respect to a predicting method in the case of executing a preceding value prediction. A preceding value predictive coding circuit shown in FIG. 1 comprises: an input terminal 601; a subtracting circuit 602; a non-linear quantization circuit 603; a local decoding circuit 604; and an output terminal 609. A digitized information signal is input to the input terminal 601. An input value a is input to a plus terminal of the subtracting circuit 602. A preceding value predictive value c formed by the local decoding circuit 604 is input to a minus terminal of the subtracting circuit 602. As the result of the subtraction, a prediction error b between the input value a and the preceding value predictive value c is output. In the case where the input value a is obtained from an image signal, the non-linear quantization is executed in the non-linear quantization circuit 603 by using the fact that a distribution of the prediction errors b can be substantially approximated by a Laplace distribution. A code d is output to the local decoding circuit 604 and output terminal 609. The section shown by 604 and surrounded by a broken line denotes the local decoding circuit. A representation value setting circuit 608 in the local decoding circuit outputs a representation value e corresponding to each quantization step of the code d. By adding the representation value to the preceding value predictive value c by means of an adding circuit 607, a decoded value f of the code d is output and is delayed by, for instance, one sampling period of time by a delay circuit 606. Thereafter, a prediction coefficient a (0&lt;a.ltoreq.1) is multiplied by the delayed output signal by a prediction co-efficient multiplication circuit 605 and a next preceding value predictive value c is formed. On the other hand, the decoding circuit can be realized by a construction similar to that of the local decoding circuit in the encoder shown in FIG. 6. The decoding can be executed by transmitting the value f to the output terminal.
The data which ordinarily consists of eight bits/sample (corresponding to a in FIG. 1) can be compressed into data consisting of 4 to 5 bits/ sample (corresponding to d in FIG. 1) by the predictive coding as described above and can be transmitted. The transmission bit rate can be reduced.
In recent years, the band of transmission signal has been widened more and more in association with the realization of a finer image signal. Due to such a wide band signal, it is desired to construct a coding system having a further high compression ratio. For this purpose, the assignee of the present invention has proposed a construction to realize the coding system of a further high compression ratio by a method whereby the same code is applied to a plurality of different differential values in Japanese Patent Application No. 62-36201, which has already been applied.